Gasoline stations, manufacturing plants, fleet garages, private fueling facilities, and the like typically use underground storage tanks to hold large quantities of volatile fluids which can be withdrawn from the tank and then dispensed in limited quantities at the aboveground site. Typical gasoline stations have multiple underground tanks for holding different grades of fuel, and the tanks are provided with pumps to transport the fuel to metered pumps used by the consumer to dispense the fuel into a car, truck, or the like. Underground storage tanks have the advantages of not taking up valuable aboveground space and have certain safety features, especially with respect to flammable fluids such as fuel. However, the interior of such underground tanks are prone to accumulate dirt and debris which may contaminate the fluid contained therein, and cleaning such underground tanks can be exceedingly difficult.
Underground liquid fuel tanks are one type of tank that typically accumulates troublesome amounts of sludge and debris in various ways over a period of time. For example, small objects, such as stones, may inadvertently be dropped into the tank through access openings when the tank is drained, filled, or its pumps or other accessories are serviced. In addition, impurities are introduced into the fuel by the air that must enter the tank through a vent or opening to replace the fluid as it is extracted. This air often contains moisture, dust, and microorganisms that degrade the fuel and accumulate within the tank's interior. These impurities may result in the character of the fuel changing, the interior surfaces of the tank oxidizing, and troublesome growth of microorganisms in the tank. Some microorganisms are known to thrive in gasoline and to feed on the additives typically found in gasoline and the metabolic processes of these microorganisms tend to produce in time a type of sludge within the tank which is characterized differently (and may be of substantially different composition) in different parts of the country. In the south, a red sludge typically develops that is called "red gravy" and in northern states, such as Wisconsin, the sludge has been found to be grayish white and is known as "white gravy". Over a period of time, the deposits in the lower levels of the tank become substantial and the activity and growth of the microorganisms becomes so significant that pumps, meters, dispenser filters, level probes, etc. need frequent service, repair or replacement and, in extreme cases, fuel held in the tanks is no longer acceptable for use.
Other systems and methods are known in the art for cleaning underground tanks. For example, Gorman-Rupp of Ohio markets a system in which a hose is extended down through an access opening of an underground tank and slowly moved along the bottom of the tank to draw out sludge and particles. The extracted mixture of fuel and contaminants are then pumped to a tank, such as a 55 gallon drum, for later disposal. It is believed that such a system is only marginally effective at best as the hose only removes limited amounts of sludge and debris close to its inlet and the operator cannot see into the tank to determine the location of such debris and whether the entire tank has been cleaned. Another means for cleaning underground fuel tanks involves draining the tank of fuel, filling the tank with water or nitrogen, cutting an opening large enough to permit entry of a person into the top of the tank, draining the water or venting the nitrogen out of the tank, lowering a person into the tank to manually scrub out the interior, and subsequently rewelding or sealing the top of the tank. It is believed that such systems are relatively crude, expensive, time-consuming, and often dangerous.
Other apparatus and methods are known for cleaning aboveground storage tanks but it is believed that such methods are impractical for use with underground liquid fuel storage tanks. One such known method is introducing a nozzle into an access port at the top of a tank, spraying the tank with a cleaning agent, and then draining the cleaning agent out of an access opening at the bottom of the tank. Obviously, such a method will not work with an underground storage tank. Another method is disclosed in U.S. Pat. No. 4,015,613, issued to Papworth, which discloses an apparatus having a coaxial hose that is inserted into and used to clean aboveground tanks of the type used in automobiles or the like. The coaxial hose has an outer tube for drawing fluid from the tank and an inner tube for discharging fluid back into the tank. The fuel discharged through the inner tube produces turbulence in the tank to suspend sediment and other particles in the fluid which is then drawn out through the outer tube for external filtering. While this system might be effective in cleaning small aboveground tanks, it is believed impractical for use with underground liquid fuel storage tanks. This is due to the nature of typical underground liquid fuel storage tanks which have restricted access openings (in accordance with Health and Safety Codes) of about 4 inches in diameter, bottom levels which typically lie at depths of 11 feet or more below the ground, and large volumes as such underground tanks typically have diameters of 8 or more feet and lengths of 10 to 60 feet. In order to achieve effective cleaning of such an underground liquid fuel storage tank using a coaxial hose as disclosed in Papworth, the fuel would have to be circulated at a high flow rate so that the cleaning process could be completed in a reasonable amount of time and the fuel would have to be discharged into the tank at a high velocity so that the discharged fuel would impact a greater area of the surfaces of the interior of the tank. However, the restricted access openings of 4 inches mandate that such a coaxial hose have a small diameter and thus hoses of larger diameter cannot be used to increase the flow rate through such a coaxial hose. Furthermore, the pump speed to increase the flow rate would decrease the Net Positive Suction Head (NPSH) of the pump in Papworth. If such a system were used on an underground storage tank containing volatile liquids such as gasoline which typically have very high vaporization pressures, the NPSH required from the pump to draw fuel at higher flow rates and velocities through the coaxial hose would cause the fuel to vaporize in the pump, resulting in cavitation and possible damage to the pump. It has been found that if such a coaxial hose having a diameter of less than 4 inches is used to draw fuel from an underground liquid fuel storage tank, the flow rate through an outer passage of the coaxial hose cannot exceed approximately 20 gallons per minute without causing the fuel to vaporize at pockets of high vacuum pressure in the pump. Papworth further discloses a practical embodiment in which the filtered fuel is discharged back into the tank through an inner passage of the coaxial hose which has an inside diameter of 3/8 inch and a fuel pressure of about 18 to 20 psi. It is believed that such a practical embodiment would result in the fuel having a velocity of approximately 15 to 20 feet per second. While such a flow of fuel might be effective in cleaning aboveground tanks of limited size, it is believed that underground storage tanks are typically larger and would require higher flow rates and velocities to effectively achieve agitation and filtering of the fluid contained therein in a reasonable period of time.
An important aspect of the present invention therefore lies in the discovery of an effective and efficient apparatus and method for cleaning underground liquid fuel storage tanks which employs the fuel in the tank as a cleaning agent, is capable of circulating and filtering the fluid at high flow rates, and discharges the fluid back into the tank at high velocities to dislodge and suspend particulates from the interior surfaces of the tank in the fuel for removal by the filtering process. In brief, the apparatus of the present invention comprises a main fuel pump having an inlet and outlet, a first passage-providing means defining an inflow passage communicating with the pump inlet for conducting fuel to the pump from an underground liquid fuel storage tank, and a second passage-providing means defining an outflow passage communicating with the pump outlet for returning fuel from the pump to the same underground tank. A filtering means is provided along the outflow passage for filtering fuel flowing therethrough. A jet pump means having a discharge nozzle is provided in the first passage-providing means and is directed towards the pump inlet for increasing the rate and velocity of fuel flowing through the inflow passage from the underground tank to the main pump. The jet pump means includes a fuel intake for receiving fuel from a flow-diverting conduit means which conducts a minor portion of the fuel flowing through the outflow passage, after such fuel is partially filtered by said filtering means, to the intake of the jet pump means. A hose means is used for connecting the inflow and outflow passages to the interior of the underground liquid fuel storage tank. In one embodiment of the invention, the hose means takes the form of a coaxial hose having proximal and distal ends with outer and inner passages running therebetween. At the proximal end, the outer passage is connected to the inflow and the inner passage is connected to the outflow while at the distal end, a nozzle is provided with discharge and intake ports respectively connected to the inner and outer passages. The main pump and jet pump means are operable in this embodiment to draw fuel from the tank into the intake ports at a flow rate of at least 25 gallons per minute, and preferably at 35 to 50 gallons per minute, and discharge the fluid back into the tank through the discharge ports at a velocity of at least 25 feet per second, and preferably at 35 to 50 feet per second. The coaxial hose preferably has an outer diameter of less than 4 inches and a length of at least 25 feet. By employing the jet pump means to increase the flow rate and velocity of the fuel flowing through the inflow passage and into the inlet of the main fuel pump, the NPSH required from the main fuel pump is satisfied to such a degree that the fuel will not vaporize in the pump when the apparatus is used to circulate and filter fuel at high flow rates and velocities.
In another embodiment, the hose means takes the form a first hose having a proximal end connected to the outflow and a distal end including a nozzle means for discharging fluid into the tank, and a second independent suction hose having a proximal end connected to the inflow and a distal end including inlet means for drawing fluid from the tank into the suction hose. The main pump without the aid of the jet pump in this embodiment is operable to withdraw fluid from the tank at a flow rate of at least 70 gallons per minute, and preferably at a rate of approximately 80 to 110 gallons per minute. The first and second hoses have outer diameters of less than 4 inches to fit through the restricted access openings of typical underground tanks and lengths of at least 25 feet to enable the hoses to reach the fluid at the bottom of the tank. The coaxial hose used in the first embodiment may be employed as the first hose in this embodiment to conserve materials and negate the need to transport an additional hose along with the apparatus to sites at which an underground liquid fuel storage tank is to be cleaned.
The present invention also involves the discovery of a method for cleaning and removing particulates from an underground liquid fuel storage tank containing a predetermined quantity of fuel. Such a method comprises the steps of first dislodging, suspending, and removing particulates from the underground tank by introducing a coaxial hose into the tank through an access opening until a nozzle at the lower end of the hose is located adjacent the bottom of the tank beneath the fuel level, and withdrawing fuel and particulates through the nozzle into an outer passage of the coaxial hose, circulating the fuel through filters and into a reservoir, and thereafter withdrawing filtered fuel from the reservoir and reintroducing the filtered fuel at a high velocity into the tank through an inner passage of the coaxial hose and the nozzle. The method also involves the steps of advancing the nozzle along the bottom of the tank while continuing the dislodging, suspending, and removing operation for a selected interval, and thereafter sweeping the tank by introducing a second hose into the tank through a second access opening, said second hose having an intake opening at its lower end. The intake opening is positioned in a direction facing the nozzle of the coaxial hose, and fuel is withdrawn from the tank by means of the second hose. The fuel is then filtered and introduced into the same reservoir, and the fuel is subsequently withdrawn from the reservoir and reintroduced at a high velocity into the tank through both the inner and outer passages of the coaxial hose and nozzle. The nozzle of the coaxial hose can be oscillated to dislodge and suspend particulates in the tank which are drawn by the intake of the second hose for effective removal of particles and sediment contained therein. In another embodiment, a pre-sweeping or vacuuming stage can be performed first by introducing the coaxial hose into the tank until the nozzle is located beneath the fuel level, introducing a second hose into the tank through a second access opening, the second hose having an intake opening at its lower end, positioning the intake opening in a direction facing the nozzle of the coaxial hose, withdrawing fuel from the tank by means of the second hose, then filtering the fuel and introducing the same into a reservoir, and thereafter withdrawing the fuel from the reservoir and reintroducing the fuel at high velocity into the tank through both inner and outer passages of the coaxial hose and nozzle. The intake end of the second hose can be oscillated during the vacuuming stage over the bottom of the tank to remove deposits of particulates and sediment disposed thereon.
During the operation where the coaxial hose is used to dislodge, suspend, and remove particulates from the tank, the fuel is withdrawn through the outer passage of the coaxial hose at a flow rate of at least 25 gallons per minute, and preferably at a rate of 35 to 50 gallons per minute, and the fuel is returned to the tank through the inner passage and nozzle at a velocity of at least 25 feet per second, and preferably between 35 and 50 feet per second. In the embodiments where a second hose is employed, the fuel is circulated at a flow rate of at least 75 gallons per minute, and preferably at a rate of 80 to 110 gallons per minute. Such high velocities and flow rates are effective for dislodging and suspending particulates in the fuel and filtering such particulates from the fuel within a reasonable time period.
Other advantages, objects and features of the present invention will become apparent from the specification and drawings.